Emulsion polymerization and suspension polymerization are known as methods for producing fluoropolymers. In these polymerization methods, perfluorocarbon (PFC) or chlorofluorocarbon (CFC) has been used as a polymerization medium. PFC and CFC however have a high global warming potential (GWP), and CFC also has a high ozone depletion potential. Therefore, it is preferred to reduce the amounts of these to be used. Further, since PFC and CFC are likely to adsorb on fluoropolymers, they are released and vaporized to form bubbles when the fluoropolymers are extruded, causing molding failures. Therefore, solvents alternative to PFC and CFC have been suggested.
Patent Literature 1 suggests a polymerization method that includes bringing a fluoromonomer into contact with a solvent that contains carbon; fluorine; at least one hydrogen atom; at least as many fluorine atoms as hydrogen atoms; no more than two adjacent —CH2— groups; no hydrogen atoms on any primary carbon; and no methyl (—CH3) groups, except as the grouping —CF2OCH3.
Patent Literature 2 suggests a method for producing an ethylene-tetrafluoroethylene copolymer which is polymerized using a polymerization initiator in a polymerization medium in the presence of a chain transfer agent. In the method, the polymerization medium is a saturated C3-C10 hydrofluorocarbon containing one or more fluorine atoms and hydrogen atom(s) in a number equal to or smaller than fluorine atoms, and the chain transfer agent is a saturated C3-C10 hydrocarbon or hydrofluorocarbon (excluding a saturated hydrofluorocarbons containing one or more fluorine atoms and hydrogen atom(s) in a number equal to or smaller than fluorine atoms).
Further, in the above described production methods of fluoropolymers using polymerization mediums, it is preferred that the polymerization medium is separated, recovered, and recycled after the polymerization reaction in view of economic and environmental benefits. Thus, a polymerization medium recovery method is known.
Patent Literature 3 discloses a production method of a fluorocopolymer including copolymerizing tetrafluoroethylene with a specific fluorine-containing vinyl ether using perfluoroheptane as a reaction solvent in the presence of a chain transfer agent in an amount of 0.5 to 4 mol % relative to the total amount of the added monomers, and discloses recovery of the reaction solvent after the polymerization reaction in the method.
Patent Literature 4 discloses a method for producing fluoropolymers, comprising polymerizing at least one fluorinated monomer to yield a fluoropolymer in a polymerization medium including water and a specified low-telogenic hydrofluoroether (HFE), and the method further comprises separating the fluoropolymer and water from the HFE solvent by distillation of the HFE solvent while the fluoropolymer is agglomerated, and reusing the HFE solvent for another process.